That, or just enter DEFAULT_CHATFRAME_ALPHA = 0 at the top of any Lua file in any addon you always have enabled.

This would be creating a derivative work, and you should not to this to any addon that is not licensed to allow this. In addition you would have to remember to redo this change anytime you updated the choosen addon to modify. So the others idea is better, an standalone addon that does just this for you.